Viral infection leads to the production of interferon by infected cells and stimulation of infected and uninfected neighboring cells, thus leading to the establishment of an anti-viral state in the host. This antiviral response usually prevents progression of infection. A number of viruses, including members of the flavivirus genus such as yellow fever, West Nile, tick borne encephalitis, Japanese encephalitis, and dengue, circumvent the host antiviral response. Through in vitro systems developed in our laboratory, we have found that three nonstructural proteins encoded by dengue virus (NS2A, NS4A and NS4B) block interferon response, thus compromising the host response to infection. Our characterization of NS4B has shown that this protein blocks interferon signaling by inhibiting STAT1 phosphorylation, thus preventing activation of interferon sensitive promoters and hence the establishment of an antiviral state. A deletion analysis of NS4B, has identified the sequence of the protein (aa 1-125) required for inhibition of type I interferon signaling (42, 43). Further characterization of NS4B will help us understand the precise mechanism that this protein employs to evade the host immune system thus allowing efficient replication of the virus. Our strategy is to: 1) determine the specific amino acids needed for NS4B's anti-interferon function, 2) identify NS4B cellular interactors in order to elucidate its interferon antagonistic mechanism and 3) use our findings to generate dengue viruses with mutations in their NS4B coding sequences. These mutant viruses will have a reduced ability to counteract the host antiviral response and may place us one step closer to generating attenuated viruses of potential use in vaccine development. [unreadable] [unreadable] [unreadable]